A variable gain amplifier is an amplifier stage whose gain can be adjusted using information provided by a feedback loop. There are three basic types of variable gain amplifiers that are applicable in automatic gain control: linear, exponential and polynomial. The exponential type, which is also called a linear-in-dB variable gain amplifier, is widely used because of its wide dynamic gain range. To have a wider gain range, the exponential function having an input value “2x” (i.e. exp(2x)) is preferred to be used, however, the bandwidth is always limited to be lower than 1 GHz due to the difficulty of realization, and it's not suitable for high speed serializer/deserializer (serdes) system.